When performing a photolithography in a manufacturing process of a semiconductor device, a resist coating process for forming a resist film on a surface of a semiconductor wafer (hereinafter, referred to as a “wafer”) is performed. A spin coating method is generally applied to the resist coating process. A liquid chemical for a resist film (hereinafter, referred to as a “resist” simply) is expensive, and requires a method for lowering the consumption of the resist as much as possible, while maintaining a discharge amount for ensuring a high in-plane uniformity with respect to a thickness of the resist film. That is, in order to perform the above process, it is required to detect a flow rate with a high accuracy at a region having a small flow rate. However, there is no flowmeter suitable thereto. For this reason, for example, a method which periodically adjusts a pressure of a pump using an electric balance has been used. However, such periodical adjustment is inconvenient. In addition, a bubble may occur due to gas dissolved in the resist. As a line width of a pattern becomes finer and finer, it is necessary to detect and handle a fine bubble, although the fine bubble did not cause any problem in the past when the line width of the pattern need not be finer.
In the related art, there is a known method in which a pair of ultrasonic receiver-transmitters is mounted on a peripheral portion of a pipe through which a fluid flows in order to measure a flow rate of the fluid. There is also known method in which a discharge amount is controlled by adjusting a valve in front of a discharge hole to be opened or closed based on measurement results of an ultrasonic flowmeter. Unfortunately, these methods detect a minute flow rate of 1 mL/sec or less.